1. Field of the Invention
This invention relates to rotary cutters used for cutting paper and other web materials into predetermined sheet lengths and in particular to apparatus for controlling the distance and position of one knife of a cutter relative to the other knife.
2. Description of the Prior Art
This invention relates to rotary cutters which cut a traveling web of material into predetermined sheet lengths by the coaction of a rotating knife with a bed knife that is stationary at the time of cutting. The bed knife is positioned at an angle to the direction of web travel and the rotating knife is helically shaped so that a progressive transverse square cut is made in the web.
In the simpler types of cutters the length of the cut sheet is determined by the speed of the rotating knife relative to the speed of the traveling web. To cut sheets of long length the speed of the rotating knife must be so low, that the peripheral speed of the rotating knife is less than the web speed. Therefore, during the progressive cut across the web width, the web buckles or humps between the cutter feeding means and the knives.
This buckling or humping phenomenon does not normally offer any serious problems when cutting flexible, non-brittle webs. However, certain types of web material can be satisfactorily cut only if the peripheral speed of the rotary knife is equal to or greater than the web speed. An example of these types of web materials are the impregnated papers used to make high pressure laminates, such as kraft papers impregnated with phenolic resins and heavy alpha stock papers impregnated with melamine resins. These web materials when impregnated and dried are brittle and tend to break or shatter between the infeed and the knives if the peripheral speed of the rotary knife is less than the web speed.
To overcome this problem, "miss-cut" or "skip-cut" rotary cutters have been developed which provide for cutting action to take place every second, third, fourth, etc., revolution of the rotary knife thus allowing long sheet lengths to be cut with a relatively small diameter high speed rotary knife so that the peripheral speed of the knife is equal to or greater than the web speed at all times. One such rotary cutter is disclosed in U.S. Pat. No. 3,136,194 where the rotary knife can be moved into and out of engagement with a fixed bed knife. Another type of cutter provides a rotary knife on a fixed axis and a bed knife which is moved into and out of engagement with the rotary knife.
The normal rotary cutter has one knife made of hardened steel and the other of soft or mild steel. The latter can easily be warped and dressed into position in relation to the hardened knife and any accidental contact between the knives is of little consequence. However, since many brittle web materials are abrasive it is necessary that both knives be made of hardened steel to prevent the unacceptable, rapid wear that would occur on a soft blade. However, any contact between two hardened knives results in nicks in the blades resulting in loss of cutting action and requiring that the knives be removed and reground. Since the actual clearance between blades when cutting may be as little as 0.0005 inch, it can be seen that the means for adjusting and maintaining this clearance between blades must be very precise.
Further, it has been found that different thicknesses of materials require different knife clearances to minimize difficulties during cutting. It has been found, for example, that clearances of 0.0005 to 0.001 inches are optimum for the thinnest materials and that clearances up to 0.005 are optimum for the thicker materials. Inasmuch as all ranges of material thicknesses may be normally cut on the same machine, it is necessary that a clearance adjustment means be provided which can be controlled by the machine operator to suit the particular material to be cut. Not only must the adjusting means provide repeatability without backlash, the adjusting means also must be so designed that at no time can the operator accidently adjust the knives so that they come into contact.
It is also desirable that normal wear in the knife adjusting means does not decrease the clearance between the knives and thereby possibly cause contact between the knives.